The present invention relates to an inking device for printing with greasy ink, i.e. using an ink of a viscosity greater than 50 poises, which may be used in particular in greasy ink printing machines such as the offset or typo type.
So-called "short" inking devices are already known which comprise a soft rubber inking roller, in contact with a printing or plate cylinder and rotating at the tangential speed of said latter, and a doctor made of hard material, of very small diameter and connectable to a rod, which is applied under pressure against the inking roller and rotates in the same direction as said latter, i.e. the displacements of the two rollers are made in opposite direction in their zone of contact. A mass of ink is housed and contained in the space above the zone of contact of the two cylinders and the greasy ink is thus laminated on passage therebetween.
Theory and experience have shown that by applying the doctor under strong pressure against the inking roller, and by causing it to rotate in the direction of the inking roller, it was possible to laminate the ink and to obtain a film of ink of very small thickness, constant over the whole length of the inking roller, which conditions are necessary for obtaining a good quality print.
Furthermore, offset printing machines are usually provided with a dampening device which applies on the side surface of the previously inked inking roller, i.e. downstream of the ink trough, a film of a dampening liquid, conventionally water, which is conveyed jointly with the ink to the zone of contact between the inking roller and the plate cylinder.
It has been ascertained that with an inking device of the above-mentioned type, the dampening water may easily return to the ink trough if it is not taken along by the plate cylinder. This is particularly the case when particularly hydrophobic inks which absorb very little water are used for printing. In fact, the water which remains on the peripheral surface of the inking roller downstream of the zone of contact with the plate cylinder, and which is consequently returned by this inking roller into the ink trough, is normally combined with the ink to form a stable emulsion which is foreseen by the ink manufacturers and is therefore not a hindrance. On the contrary, if the capacity of emulsion is exceeded, the excess water remains in the form of drops disturbing the distribution of the ink in the ink trough: these heterogeneities of distribution are shown by streaks in the printing.
The return of water is less frequent in the conventional inking devices which comprise a plurality of rollers: the water here in fact meets a large surface and can evaporate under the effect of the heat and the ventilation provoked by the rotation of the rollers. However, these inking devices present such drawbacks that the "short" inking devices are preferred, despite the difficulty resulting from the dampening which has been set forth hereinabove.